1. Field of the Invention
The present invention relates to power semiconductor devices.
2. Related Art
There is a demand for a more efficient and smaller power supply circuit such as a switching power supply, an inverter, etc. One way for achieving this would be a decrease in power loss in a power switching element in a power supply circuit, specifically a decrease in conduction loss achieved by a decrease in ON voltage of a power switching element, or a decrease in switching loss achieved by a decrease in switching charge. With such a decrease in power loss, it is possible to increase the circuit efficiency. In addition, it is possible to decrease the size of a power supply by decreasing the size of passive elements in a circuit by increasing the frequency of the switching frequency. Conventionally, hard switching, in which a high voltage and a high current are applied to a switching element, has been performed. However, in such cases, a problem of noise at the time of performing a switching operation arises, which is caused by increasing the switching frequency.
In order to solve this problem, a soft switching circuit technique has been developed as a means for achieving a higher efficiency and a smaller size without increasing noise, as disclosed in Japanese Patent Laid-Open Publication No. 2000-156978. In soft switching, the switching loss can be made negligibly small, and the noise at the time of switching can also be reduced by performing a switching operation under a condition where almost no voltage is applied.
In a conventional power supply circuit, a vertical power MOSFET has been widely used as a switching device. Even if the switching system is changed to a soft switching system, a vertical power MOSFET is one of the most widely used switching elements. However, a conventionally used power MOSFET cannot offer satisfactorily outstanding performance because of the switching system being different, since the performance expected for an element is different in the soft switching.